1. Field of the Invention
The present invention is generally related to a sprue for a lost foam casting system and, more particularly, to a sprue that directs the flow of molten metal in a preferred direction from bottom to top in order to improve the quality of a metal casting made by a high pressure casting system.
2. Description of the Prior Art
The lost foam casting process, using evaporative foam patterns, is well known to those skilled in the art. U.S. Pat. No. 4,721,149, which issued to Hesterberg et al on Jan. 26, 1988, discloses a lost foam casting system with a high yield sprue. The system includes a sprue having a receiving portion for receiving molten metal from a source, a distribution portion with a plurality of radial fingers for distributing the molten metal radially outwardly, and a lower section having a plurality of feed passages receiving molten metal from the respective radial fingers and interconnected by thin flat support walls providing a relatively rigid structure. Work pieces and in-gates are connected to the flat exterior surfaces of the lower section of the sprue and fed with molten metal from respective feed passages. The structure provides pattern assembly rigidity, excellent dimensional control, high casting yield, high ratio of casting weight to gating/sprue weight, high surface area to volume ratio facilitating escape of vapor and organic effluents, ease of pattern attachment by large outer flat sprue surfaces, accessibility of casting for ease in removal from the sprue, low total volume for reduction of buoyancy, and faster metal fill rates and faster cooling.
U.S. Pat. No. 5,518,060, which issued to Cleary et al on May 21, 1996, discloses a method of producing polymeric patterns for use in evaporable foam casting. A positive three-dimensional model of the metal part to be cast is made by a layered prototyping process in which layers of sheet material are bonded in superimposed relation and the contour of the part to be cast is cut into each layer as it is applied to preceding layers to provide the model. A metal, such as copper, is then deposited on the working surface of the model to provide a rigid self-supporting shell having a surface which is the negative image of the part to be cast. The shell, after separation from the model, is then mounted in a die casting mold with the negative surface bordering a die cavity. Beads of a polymeric material, such as polystyrene, are introduced into the die cavity and heated to fuse the beads and provide a foam pattern which is identical in configuration to the metal part to be ultimately cast.
U.S. Pat. No. 4,802,447, which issued to Corbett on Feb. 7, 1989, discloses a foam pattern for an engine cylinder block. A polystyrene foam pattern is provided for the cylinder block of a crankcase compression two-cycle engine having transfer passages formed in the block. The pattern includes a head-end component and a crankcase end component mating with each other. The mating surfaces extend through the transfer ports, transfer passages, and exhaust ports to allow the formation of complex passages with die cast pattern components.
U.S. Pat. No. 4,966,220, which issued to Hesterberg et al on Oct. 30, 1990, discloses an evaporable foam casting system utilizing a hypereutectic aluminum-silicon alloy. The method of casting utilizes an evaporable foam system with a hypereutectic aluminum silicon alloy. The molten alloy is introduced into a mold in contact with an evaporable foam pattern formed of polystyrene, or the like. The heat of the molten alloy will decompose and vaporize the pattern and the vapor will enter the interstices of the surrounding sand, while the molten alloy will fill the void caused by the vaporization of the pattern. By casting the molten alloy into contact with the evaporable foam material, a more uniform distribution of primary silicon is obtained in the cast alloy and the heat of crystallization caused by precipitation of silicon crystals on solidification of the alloy will temporarily slow the solidification rate of the alloy, thus increasing the time for elimination of pattern residue vapors from the molten alloy.
U.S. Pat. No. 5,038,847, which issued to Donahue et al on Aug. 13, 1991, discloses an evaporable foam pattern for use in casting a crankshaft. A pattern for use in casting a rotatable shaft, such as a crankshaft for an internal combustion engine is disclosed. The pattern includes an evaporable foam pattern section composed of a material such as polystyrene and having a configuration conforming to the crankshaft to be cast. The evaporable foam pattern section includes a plurality of cranks connected by bearing areas, and tubular metal inserts formed of bearing quality steel which are disposed around each bearing area and around the pin areas of the cranks. In the casting process, the pattern is placed in a mold and surrounded with a finely divided material such as sand. When molten ferrous metal is fed into contact with the pattern, the pattern will vaporize with the vapor passing into the interstices of the sand while the molten ferrous metal will occupy the void created by the vaporized foam to produce a cast crankshaft having bearing quality steel inserts at the bearing and pin areas.
U.S. Pat. No. 5,355,930, which issued to Donahue et al on Oct. 18, 1994, discloses a method of expendable pattern casting of hypereutectic aluminum-silicon alloys using sand with specific thermal properties. A method of producing a casting utilizing an expendable polymeric foam pattern along with unbonded sand having specific thermal properties is disclosed. The pattern, formed of a material such as polystyrene, has a configuration corresponding to that of the article to be cast. The pattern is placed within an outer flask and unbonded sand surrounds the pattern as well as filling the cavities in the pattern. The sand has a specific heat diffusivity greater than a preselected magnitude. The molten hypereutectic aluminum silicon alloy is fed into the flask in contact with the pattern causing the pattern to vaporize with the vapor being entrapped within the interstices of the sand while the molten metal fills the space initially occupied by the foam pattern to produce a cast article. The thermal properties of the sand reduces the particle size of the precipitated primary silicon particles in the casting, thereby increasing the machinability of the casting.
U.S. Pat. No. 5,355,931, which issued to Donahue et al on Oct. 18, 1994, discloses a method of expendable pattern casting using sand with specific thermal properties. A method of producing dimensionally predictable metal castings utilizing an expendable polymeric foam pattern along with unbonded sand having specific thermal properties is disclosed. The pattern, formed of a material such as polystyrene, has a configuration corresponding to that of the article to be cast. The pattern is placed within an outer flask and unbonded sand surrounds the pattern as well as filling the cavities in the pattern. The sand has a linear expansion of less than a preselected magnitude, a heat diffusivity greater than a preselected magnitude, an AFS grain fineness number within a preselected range, and an AFS base permeability number within a preselected range. A molten metal, such as an aluminum alloy or a ferrous alloy, is fed into the mold in contact with a pattern causing the pattern to vaporize with the vapor being entrapped within the interstices of the sand while the molten metal fills the space initially occupied by the foam pattern to produce a cast article. The physical properties of the sand enable articles to be cast having more precise and predicable tolerances.
U.S. Pat. No. 5,960,851, which issued to Donahue on Oct. 5, 1999, discloses a method of lost foam casting of aluminum-silicon alloys. An improved method of lost foam casting of aluminum silicon alloys utilizes a pattern formed of an expendable polymeric foam having a decomposition temperature less than 300° C., and a heat of decomposition less than 600 Joules per gram. The foam pattern preferably has a heat of fusion less than 60 Joules per gram and a bulk density in the range of one to four pounds per cubic foot. The lost foam casting procedure has a particular use when casting a hypereutectic aluminum silicon alloys containing from 16 to 30% silicon, and eliminates the “liquid styrene” defect which occurs when casting such alloys in a lost foam process utilizing conventional polystyrene foam patterns. When casting hypoeutectic aluminum-silicon alloys containing from 5% to 8% silicon, the method eliminates the “fold” defect.
U.S. patent application Ser. No. 09/843,184, which was filed on Apr. 26, 2001, by Donahue et al, discloses method and apparatus for casting of metal articles using external pressure. A method and apparatus for casting of metal articles using external pressure and having particular application to lost foam casting of metal articles is disclosed. A polymeric foam pattern having a configuration corresponding to an article to be cast is placed in an outer flask and the pattern is connected through a polymeric foam gating system to a pouring cup located at the upper end of the flask. The pouring cup has a volume equal to 5% to 75% of the combined volume of the gating system and pattern. A finely divided inert material, such as sand, is placed in the flask surrounding the pattern and fills the internal cavities within the pattern. The flask containing the pattern is then positioned in an outer pressure vessel having a removable lid and a molten metal is fed into the pouring cup. The lid on the pressure vessel is closed and an external gaseous pressure is applied to the molten metal in the pouring cup as the molten metal feeds through the gating system to the pattern and progressively ablates the polymeric foam material, with the gaseous products of decomposition passing into the interstices of the sand and the molten metal filling the void created by decomposition of the foam. By applying pressure to the molten metal during filling, the molten metal front is more stable and fewer casting defects arise.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
Those skilled in the art who practice the procedures of lost foam casting must be aware of certain potentially disadvantageous characteristics of the process and must design the sprue system and patterns for the article to be cast in order to avoid certain potential problems. For example, the lost foam pattern must be designed in such a way that the pyrolized products resulting from the melting and vaporization of the polystyrene material be accommodated in such a way that they are not excessively entrained within the cast article. In addition, cast articles that contain thick portions can result in a problem relating to voids caused by contraction as the thick portions of the cast article cool. If the in-gates are not sufficiently sized to maintain continual availability of molten metal as the thick portions of the cast article solidify, the resulting contraction of the thick portion can create voids, related to the density difference between liquid and solid metal, that are not able to be subsequently filled by a ready supply of molten metal.
It would therefore be significantly beneficial if an improved sprue could be provided for use in conjunction with lost foam casting of metal articles, particularly when high pressure is introduced during the casting process.